Source of markup ghosts:

1. proprietary markup introduced by browser vendors
2. specs which didn*t see uptake like HTML3
3. use case specific markup on devices like WebTV or early smartphones

Some old tags:

• <audioscope> to display the sound over time. A demo https://www.youtube.com/watch?v=tM7YVhar2Dg
• <au> for authors
• <blackface> for double-weight boldface... in HTML 🤡
• <center> oh shit
• <font> and color attributes
• dingbat attribute to embed emojis, extended unicode entities, icon fonts and custom things. Now it is used as < for example and does not rely on the attribute anymore.
• <multicols> ....
• <person> in HTML 3, but it opens the door for so many other tags. Now instead we can rely on <a rel="author"> or with me

and more